This invention relates to fuel injectors for diesel engines.
FIG. 9 is illustrative of prior art diesel fuel injectors incorporating therein an intensifier piston 410. The intensifier piston 410 cooperates with the adjacent parts of the housing 412 to create an upper chamber 414, a middle chamber 416, and a metering chamber 418. The intensifier piston further includes an internal passage 430 having inserted therein a capillary or laminar flow restrictor 432. The intensifier system 40 may be fabricated of unitary construction or equivalently may include an upper member 436 that is pressfit into a lower number 438 to define a stepped outer contour of the intensifier piston. The intensifier piston is movably received within a stepped bore 439 provided by the housing 412. The excess fuel that is received within the upper chamber 414 is dumped in correspondence with motion of the intensifier piston 410 through the passage 430 through the middle chamber 416 and passage 434 to a port 440.
Prior to injecting fuel from the injector 400, a predetermined quantity of fuel is received into the metering chamber 418 in a known manner. After injection, that is after the intensifier piston 410 has compressed and injected the fuel within the metering chamber 418 to a determinably high pressure level, this high pressure fuel is dumped through passages 442, 444 into the middle chamber 416 and through the cooperating passage 434 to the port 440.
To increase the efficiency of the operation of the fuel injector 400 it is desirable to prohibit fuel from leaking into the middle chamber. It can be seen that if a sufficient quantity of fuel resides within the middle chamber 416 additional work must be expended to compress this fuel, thus reducing the efficiency of operation of the intensifier piston. During normal operation, the intensifier piston 410 is caused to move downward within the stepped bore 439 by introducing into the upper chamber 414 a fluid having a high pressure level. It can be seen, however, that this high pressure fuel which is also received within passage 430 has a tendency to leak between the sides of the piston and cooperating housing parts and flow into the middle chamber. This leakage from the passage 430 into the middle chamber is shown by the arrow designated as 450. In addition, as the intensifier piston is moving downward, thus compressing the fuel within the metering chamber 418, the fluid pressure level within the passage 442 increases substantially and consequently another leakage path exists, as designated by the arrow 460, which permits fuel within the metering chamber to similarly leak into the middle chamber 416. If the leakage of fuel into the middle chamber 416 is not controlled, the efficiency of operation of the injector decreases, unnecessary heating is produced and larger injectors are required to compensate for this inherent inefficiency.
Prior art fuel injectors are further characterized by premature failure due to gall-up of the intensifier piston. Prior art fuel injectors are fabricated by employing costly and precise machining tolerances to control the concentricity of the bores defining the upper and lower parts of the stepped bore 439, and the cylindricity of the intensifier piston 410. Precise machining tolerances are required to attempt to maintain the intensifier piston in an axial alignment with the stepped bore. If the intensifier piston becomes skewed relative to the stepped bore it will gall-up, become seized, and fail.
It is therefore an object of the present invention to control both the amount and direction of the leakage flow and to minimize the leakage flowing into the middle chamber of an injector. In addition, it is an object of the present invention to control the leakage flow by eliminating fluid passages through the intensifier piston. It is another object of the present invention to improve the efficiency of operation of a diesel fuel injector having an intensifier piston therein and to do so in an uncomplicated manner. A further object of the invention is to provide a fuel injector which does not require exacting machining tolerances and a fuel injector having an intensifier piston that does not tend to gall-up. A feature of the present invention which permits the control of the leakage flow is the incorporation of a piston annulus which is always connected to a low pressure vent therein permitting the collection and diversion of the leakage fuel away from the middle chamber. An additional feature of the present invention is a two member intensifier piston that permits translational movement between its members. This translational degree of freedom permits each member to move freely within a corresponding part of a stepped bore without gall-up.
A further feature of the invention is to provide a diesel fuel injector having dump ports which accomplish the following:
(1) Rapidly terminate injection for good engine performance, but not too rapidly that the injection fuel pressure drops below the combustion pressure before the nozzle needle closes, which would cause blowback of combustion gases.
(2) Limit the nozzle needle closing impact velocity to an acceptable limit for durable performance.
(3) Smoothly dump excess pump flow to prevent excessively high pressure and cavitation.
(4) Decelerate the intensifier piston so that the piston impact velocity after injection is tolerable for durable performance.
(5) Control line dynamics so that the intensifier piston will remain in its reference (bottom) position after injection until the start of the next metering cycle for repeatable, predictable injection quantities.
These requirements are met by having two separate dumps which have either annular or annulus-to-annulus dumps which open extremely fast and, in addition, each dump has an orifice in series to limit the amount of flow therethrough. With the combination of a variable orifice (annulus) and a fixed orifice, the rapid termination of injection is accomplished. The fixed orifice is used to control the impact velocity and to stabilize the injector pressure and intensifier piston. The dump flow area is shown graphically in FIG. 10.
More specifically, the invention comprises a diesel fuel injector having metering and injection modes of operation. The fuel injector comprises a housing having a plurality of fuel carrying passages therein. These fuel carrying passages terminate at a first port that is adapted to receive pressurized fuel from a first fuel source and in a second port which is similarly adapted to receive pressurized fuel from a second fuel source. The housing further includes a stepped bore defining therein an upper bore, a middle bore, and a lower bore situated below the middle bore. The housing further includes a first dump port means located within the upper bore and a second dump port means located within the lower bore. The first dump port means comprising an annulus and an orifice in series. The fuel injector further includes an intensifier piston reciprocatively situated within the stepped bore defining, in cooperation with the housing, a plurality of variable volume chambers such as an upper or primary chamber, an inner or middle chamber, and a lower or metering chamber. The intensifier piston comprises a cylindrical upper member having walls that are fluid tight and reciprocatively received within a mating wall of the upper bore and further includes a first pressure receiving surface defining the lower extremes of the upper chamber. The upper member further includes a bottom defining the upper extreme of the middle chamber. In addition, the length of the upper member is sized so that the first pressure receiving surface will uncover the first dump port means in correspondence with the piston travel. The intensifier piston further includes a lower member, having an outer cylindrical wall, which is operatively connected to the upper member. The lower member further includes a bottom defining a second pressure receiving surface which forms the upper extreme of the lower or metering chamber. As mentioned the lower member further includes the second dump port means that comprises a lower annulus and a second dump orifice combination. The lower annulus is located intermediate the middle and metering chambers. The lower annulus is maintained in constant fluid communication with the second dump orifice which in turn is connected to the second fuel source. The fuel injector further includes first check valve means connecting the metering chamber with the second port for permitting fuel flow into the metering chamber from the second port during the metering mode of operation and for preventing backflow during the injection mode of operation. The diesel fuel injector further includes a pressure activated nozzle means extending from the housing and maintained in fluid communication with the metering chamber for injecting fuel therefrom in correspondence with the pressure within the metering chamber. The housing of the fuel injector further includes a first passage which connects the metering chamber with the first dump port means, a second passage which connects the first dump port means to the second port wherein a second check valve means is situated within the second passage for permitting flow of fuel from the first dump port means to the second port and for preventing reverse flow thereto.
For the present injector to operate properly, it is desirable, therefore, that additional fuel does not enter the middle chamber. Consequently, it is necessary to design the injector to have a minimum number of leakage paths between the higher pressure upper chamber or the metering chamber and the middle chamber. Practically it is unlikely to construct a piston which does not have any leakage paths. Consequently, it is necessary to control the leakage paths to prevent fuel from entering the middle chamber. This is accomplished by providing the previously described low level pressure secondary dump port means intermediate the middle chamber and the metering chamber. It can be seen that the lower annulus and orifice in series provides two functions. The first is to dump the high pressure fuel within the metering chamber at the end of the injection cycle and secondly, to provide a leakage path for fuel leakage from the metering chamber away from the inner chamber during the injection mode of operation.